The present invention relates to a filtration method of copper electrolyte, and particularly, relates to a technique to improve filtration treatment efficiency with powdery activated carbon.
It has been conventionally known that electrolytic by-products and dirt in copper electrolyte have significant effects on physical and other properties of electrodeposits obtained by an electrolytic treatment in copper plating, copper electroforming, and so forth. Accordingly, such unnecessary electrolytic by-products and dirt in copper electrolyte are removed by a filtration method, a so-called precoating method with a filtering aid.
In this precoating method, a filtering aid such as diatomaceous earth and pearlite is precoated to a filter element such as filter cloth and a metallic screen. Copper electrolyte is passed thereto, thus depositing electrolytic by-products and dirt in the electrolyte to a surface of a precoated layer as filter cake, for removal. Filtration may be performed highly efficiently without clogging over a long period, and is extremely convenient even when a large volume of electrolyte is treated, so that this filtration method is widely used. The method also has an advantage in that filtration may be performed in accordance with the size and so forth of an object to be removed by appropriately selecting the type, particle size and so forth of a filtering agent.
However, this precoating method has a limitation on filtering minute electrolytic by-products and dirt of 0.5 xcexcm or less. Also, in removing minute electrolytic by-products and so forth by reducing the particle size of a filtering agent, filtration efficiency sharply declines; in other words, permeation of electrolyte worsens, which is not practically preferable.
As a method to efficiently remove such minute electrolytic by-products and dirt, a filtration method with activated carbon is known. Since activated carbon has an excellent adsorption property, it is suitable for filtering and removing minute electrolytic by-products and so forth. Moreover, when copper electrolyte is treated with activated carbon, the physical property of obtained copper electrodeposits may be controlled, so that activated carbon is often used in a copper electrolytic plating.
As the filtration method with activated carbon, a so-called granular activated carbon having the particle size of about 5 to 60 mesh (2 to 0.25 mm) is filled in a cylindrical treatment column provided with a perforated plate inside, and copper electrolyte is passed through the treatment column for treatment. According to the filtration method with activated carbon, minute electrolytic by-products and dirt may be removed. However, as the electrolyte is continuously passed through, the activated carbon in the treatment column forms sections so that the electrolyte may easily pass through, generating a so-called biased flow, and contact between the granular activated carbon and the copper electrolyte becomes insignificant. Moreover, since activated carbon of a large particle size is used, a contact area with copper electrolyte is small, so that filtration efficiency is not considered satisfactory.
Therefore, in order to make the filtration treatment with activated carbon reliable, a great amount of activated carbon had to be filled in to extend contact time between copper electrolyte and activated carbon. This would lead a cost increase in a copper electrolytic plating, and it is not preferable since the treatment volume of electrolyte increases. Moreover, as a method to increase contact area between copper electrolyte and activated carbon of a small particle size, a so-called powdery activated carbon may also be considered for use. In this case, it is ideal to use activated carbon of a smaller particle size in order to enlarge a contact area. However, as a particle size becomes smaller, powdery activated carbon is likely to be mixed into copper electrolyte, and the mixed powdery activated carbon provides effects on the quality of copper electrodeposits. Moreover, in case of powdery activated carbon, unlike granular activated carbon, it is difficult to fill the powdery activated carbon in a treatment column provided with a perforated plate and to continuously let copper electrolyte pass through, for an application. Thus, a batch processing method has to be taken. This is not preferable as an application to a step for a continuous copper electrolytic plating.
The present invention is made under the above-noted circumstances as a background, and is to provide a filtration method of copper electrolyte that can remove minute electrolytic by-products and dirt and may also significantly improve filtration efficiency by improving a conventional filtration method, the so-called precoating method.
In order to solve the problems, in a filtration method of copper electrolyte to remove electrolytic by-products and dirt which affect copper electrolysis, by passing the copper electrolyte through a filter element precoated with a filtering aid, in the invention, a precoated layer of a filtering aid is formed on a filter element in advance. Activated carbon preliminary treatment solution containing powdery activated carbon is passed through the filter element formed with the precoated layer, and is also circulated until no powdery activated carbon leaks from an outlet of the filter element, thus forming a powdery activated carbon layer on the precoated layer. Subsequently, copper electrolyte is passed through for filtration.
According to the present invention, minute electrolytic by-products and dirt contained in copper electrolyte may be surely removed for filtration without mixing powdery activated carbon into copper electrolyte. Moreover, as powdery activated carbon is used, a contact surface area of the activated carbon sharply increases. A flow velocity may be reduced even at a large flow rate in volume. A long contact time may be ensured, and filtration efficiency may sharply improve.
A characteristic of the present invention is to form a powdery activated carbon layer further on a precoated layer by circulating the activated carbon preliminary treatment solution containing powdery activated carbon. The precoated layer of the filtering aid formed on the filter element has fine mesh, a so-called strainer, to let copper electrolyte pass through. However, in the present invention, powdery activated carbon is deposited on the strainer formed of the filtering aid, and the powdery activated carbon layer is formed on the precoated layer finally. When powdery activated carbon is passed through the precoated layer, a phenomenon is initially seen in which most of the powdery activated carbon passes through the precoated layer and leaks out from an outlet of the filter element. However, as the circulation is being repeated, the powdery activated carbon gradually fills up the strainer of the precoated layer and finally stops leaking therefrom. As the circulation is further repeated, a powdery activated carbon layer through which only solution can pass is formed on the precoated layer.
The filtering aid relating to the present invention may be a commonly known filtering aid. For instance, diatomaceous earth, pearlite, cellulose, and so forth may be used. Moreover, a filter element relating to the present invention may be filter cloth and a metallic screen, or other porous elements as long as a filtering aid may be precoated thereto and solution can pass through it by adding pressure to the solution. Additionally, the activated carbon preliminary treatment solution relating to the present invention is not particularly limited in its composition. For instance, copper electrolyte as a filtering object may be directly used, and the copper electrolyte may be diluted for use. In short, in case of filtering by passing copper electrolyte after a powdery activated carbon layer is formed, any treatment solution may be used as long as the activated carbon preliminary treatment solution provides no effects on a copper electrolytic plating by being mixed into the copper electrolyte.
In the filtration method of copper electrolyte relating to the present invention, filtration efficiency may improve further if a precoated layer and a powdery activated carbon layer are alternately deposited. In this case, a precoated layer is formed as the lowermost layer, and a powdery activated carbon layer and, moreover, a precoated layer may be sequentially deposited thereon. The number of layers and the thickness thereof may be appropriately decided in consideration of filtration efficiency, in other words, passage of copper electrolyte, and size, type, quantity and so forth of electrolytic by-products and dirt being removed.
The powdery activated carbon for use in the filtration method of copper electrolyte relating to the present invention is preferably the one having 50 mesh (0.287 mm) or less in particle size, and more preferably the one having 50 to 200 mesh (0.074 to 0.287 mm) in particle size. The powdery activated carbon having 50 mesh or less particle size is powdery activated carbon that can pass through standard screen of 50 mesh. With powdery activated carbon of larger than 50 mesh in particle size, a surface area of an individual activated carbon particle becomes small, and filtration efficiency does not improve much. Moreover, in consideration of filtration efficiency, costs and so forth, 70 to 170 mesh activated carbon is preferable for actual operations. A term, powdery activated carbon, in the present invention is used above and below for not only the so-called powdery activated carbon but also granular activated carbon obtained by crushing or granulation.
Additionally, the thickness of a powdery activated carbon layer formed by the filtration method of copper electrolyte relating to the present invention is preferably 2 to 20 mm. When it is less than 2 mm, the removal of minute electrolytic by-products and dirt tends to become incomplete. When it is thicker than 20 mm, filtration efficiency, in other words, the passage of copper electrolyte becomes poor, and it is also undesirably costly.
Moreover, the filtering aid for use in the present invention is made of diatomaceous earth of 3 to 40 xcexcm particle size. It is preferable to use the filtering aid in which diatomaceous earth of 3 to 15 xcexcm particle size is mixed with diatomaceous earth of 16 to 40 xcexcm particle size at the ratio of 7:3. By applying diatomaceous earth with the mixture of such particle sizes and mixing ratios, a powdery activated carbon layer may be easily formed, and filtration efficiency may improve significantly.
According to the filtration method of copper electrolyte of the present invention described above, when electrolytic plating are carried out with additives, for instance, organic matters such as glue and gelatin, cellulose, ether, thiourea and so forth, to control the physical property of copper electrodeposits, decomposition products of the additives may be efficiently removed, and clean copper electrolyte may be reproduced. Generally, these additives are frequently decomposed by being added to copper electrolyte or by an electrolytic treatment, and the decomposition products become extremely minute. The decomposition products tend to affect the physical and other properties of copper electrodeposits when decomposition products are left in a large quantity in electrolyte. However, according to the present invention, even if the additives are added to continue a copper electrolytic plating, copper electrodeposits having constant physical property may be manufactured with stability.